The United States Postal Service needs a reason for more people to use its services now that hardly anyone sends personal letters anymore, and it’s hoping that same-day delivery can be that differentiator.

According to a report by the New York Daily News, the USPS plans to bring same-day delivery to New York, expanding a test program that’s already in place in San Francisco. The Daily News also pointed to the regulatory filing (PDF) in which the USPS said its Metro Post same-day delivery service, which has been in testing since last December, would continue for another year and in the new market.

The Bloomberg report also drew a direct line between this expansion and the USPS’s agreement with Amazon (NASDAQ: AMZN) to deliver that company’s packages on Sundays, in that the USPS desperately needs a new business model in order to stop hemorrhaging cash, and striking deals with individual retailers may lead to a better model.

However, there are still plenty of hurdles to overcome. The USPS’s primary value is still tied to its commitment to all parts of the United States, both urban and rural; that is, the USPS delivers to both apartments in Harlem and remote ranches in Montana. While cities are well-acquainted with the idea of subsidizing less-dense places, it’s unclear just how much money the USPS can make from same-day delivery, and how good a test San Francisco and New York provide for Metro Post’s long-term viability.

San Francisco has a small geographic footprint, only seven miles by seven miles, and a long history of employing bicycle couriers for same-day deliveries. The USPS’s filing does not shed much light on its specific plans for New York, but it’s likely the Metro Post service will be rolled out on a borough-by-borough, or an even-more-limited basis in order to segment the city into manageable chunks.

So if the program is successful in the two densest cities in the country, covering relatively small land areas, and both of them technology-friendly, that might prove the concept can work in that type of city, and private business can feel free to take up the mantle and compete. That’s good for New Yorkers and San Franciscans.

But the USPS isn’t aiming very high by going to those two cities. If it thinks it can beat United Parcel Service (NYSE: UPS) and FedEx (NYSE: FDX) on same-day delivery in big cities, thus subsidizing its other operations, it will lose in those big cities, eventually, just as it loses in regular package delivery at the moment. Rather, the USPS needs to win and make money where those two companies don’t want to be, in areas with sparse, dispersed populations, and hope there are enough of them across the country to make the effort worthwhile.

Surely, it’s an unsolved puzzle, because if companies knew how to do it, they’d have done it already, but serving those areas plays to the USPS’s established strengths: ubiquity of post office operations and its commitment to delivery in all parts of the United States. That is to say, every company in the industry would love to lead in same-day delivery in New York City, but how many want to lead in Winfield, KS? More importantly, the USPS may be the only entity that can lead in Winfield and the thousands of towns like it.